Amalgam is one of the oldest dental restorative materials which has been used for more than 150 years. Dental amalgam can release minute amounts of elementary mercury, a heavy metal whose toxicity at high intake levels is well-established. People with amalgam filling were found to have higher concentrations of mercury in various tissues including blood, urine, kidney and brain compared to those without. The health of human beings may be affected by the presence of mercury in their body.
As the popularity of dental amalgam has been reduced due to its poor aesthetic value and mercury content, many alternative materials for dental restoration have been developed. These materials include composites, glass ionomers and resin modified glass ionomer cements. They have not been as effective as dental amalgam in providing a durable and long-lasting restoration, especially in larger tooth cavity. Advances in technology have resulted in improvements that have expanded their use.
Dentists have been using a combination of composites and sealants to treat incipient lesions and thereby conserve tooth structure. However, some of the composites may not be appropriate as substitutes for restorations in stress-bearing areas. Some composites that sensitive to moisture and lack of resistance to chewing stress may also compromise their durability.
Glass ionomers are used as dental fillings because they are capable of releasing fluoride and requiring minimal drilling. The procedure of glass ionomers is quick, painless and may not require local anesthesia and the restoration is fairly aesthetic. However, the limitations of this material include low tensile strength, low impact and fracture resistance and degradation qualities. It is not appropriate to be used in the occlusal surfaces of adult teeth and stress-bearing restorations, and situations where moisture control is difficult.
Resin modified glass ionomer cement is a combination of glass ionomer and composite resin. It holds up better than glass ionomer, but not as well as composite resin, and is not recommended for biting surfaces of adult teeth. In general, resin modified glass-ionomer cements can achieve a better aesthetic result than conventional glass ionomers, but not as good as pure composites.
Successful development of resin systems for dental composite should be directed to development of monomers or oligomers which can exhibit the fracture toughness of elastomer-modified resins, while at the same time maintaining a high degree of environmental durability in the oral environment. Sterrett et al (1987) synthesized and evaluated the fracture toughness of a series of urethane toughened methacrylate resin (UTM). The polyol used in formulation of UTM resins serves as both oligomer backbone and urethane soft segment. The structure and functionality of the polyol have a direct effect on the toughness and mechanical properties of the resultant resin. Their result indicated UTM resins based on polyfunctional polyols tend to possess improved toughness. Since then various work has been reported on the development of dental polymer using polyols.
Composite resins has evolved and its refinement has always been focused on the fillers (nano-filler) and its interface, however the matrix system has remained unchanged. The matrix has been predominated by methacrylate-based resin. The common monomers used in resin composite are bisphenol-A glycidyl dimethacrylate (Bis-GMA), urethane dimethacrylate (UDMA) as base monomer or oligomer and triethylene glycol dimethacrylate (TEGDMA) and hydroxyethyl methacrylate (HEMA) as active diluents.
There is a U.K. Patent No. GB1465897 relating to a dental composition. This composition comprises a polymerizable hygienically unsaturated groups and being the reaction product of a urethane polymerize and an ester of acrylic or acrylic acid with a hydroxy Balkan of at least 2 carbon atoms. A Japanese Patent No. JP2000175941 also discloses a repair composition for a denture base which comprises methacrylate, polyurethane, filler and polymerization accelerator. This denture base aims to provide sufficient allowance time for operation, an excellent operability and lessen the refailure of a repair point due to its moderate elasticity.
These methacrylate based resins have a shortcoming in their degree of polymerization being not high enough, and there is also a lack in crosslinking density and thus, compromised their mechanical properties, and increase water uptake and swelling, lead to increase elution of chemicals components. These drawbacks cause poor biocompatibility, reduced of durability and unsatisfied of clinical performance of resin composite restorations.
Numerous studies have been focused on the use of polymeric composites in dental materials. U.S. Pat. No. 6,383,279 also relates to a dental impression composition comprising functionalized polyether derivative bonded through hydroxyl groups of polyether polyol compounds. The level of cyclic oligomeric polyethers is lower than 5.0 wt % and the functionalized polyether derivative contain an aziridino group. This composition provides a poor demouldability of the gypsum model after forming the cast of the impression.
Another U.S. Patent No. US2005059752 relates to a stable, cationically polymerizable dental composition having high filler contents. This composition comprises a compound of UV-and cationically reactive oxirane-functionalized silicone, a filler of silicone oxide, a polymer or copolymer having amine group, a cationic photoinitiator and optionally a photosensitizer.
There is also a dental prosthetic made up of hard, rigid and non-hydrophilic polyurethane (PU) elastomer disclosed in U.S. Pat. No. 4,791,156. The dental prosthetic comprises an organic polyisocyanate, branched chain polyester polyols. It possesses several highly desirable properties such as high heat distortion temperature and excellent impact resistance.
The patented technologies reveal numerous types of dental materials which comprise different types of compositions and synthesized by different methods. These discrepancies will result in a wide variety in the physical and chemical properties of the dental materials synthesized. However, none of the patented technologies provides an environmentally friendly composition of dental materials which possesses both soft segment and hard segments that impart better fatigue resistance and flexural properties. It is therefore desirable for the present invent to provide a composition or a composite resins having natural oils-derived polyurethane oligomers which is capable to overcome the drawbacks of the prior arts, such as the sensitivity to moisture, inappropriacy for stress-bearing areas, shrinkage upon curing, toxicity, low level of toughness and flexural strength and others.